When a packet comes into the main router, how does it know which subnet to give the packet to?

A campus network consisting of LANs for various departments. Router PC cs 99999977 EE TTTT Math Art To ISP TT English-I-1 French Main router Music Physics Ethernet Subnetting: how to allow a network to be split into several parts for internal use but still act like a single network to the outside When a packet comes into the main router, how does it know which subnet to give the packet to